Empirical evidence against the 'cycle time dependency' assumption

The interpretation of the 'cardiac cycle time effect', also named 'time-dependent primary bradycardia' by the Laceys, who first observed it, has been controversial in psychophysiology. Unconfounded evidence for the dependence of a vagal effect of psychological stimuli on time of stimulation within the cardiac cycle has been missing to date. An experiment in which the subjects could not anticipate the occurrence of the stimuli (short tones of a specific frequency that had to be counted) was performed. The data reduction procedure secured unambiguous interpretation of the data with respect to time-dependency or no time-dependency. No indication of any kind of cycle time dependency of the vagal effect was found.